Zircon (ZrSiO.sub.4) and zirconia (ZrO.sub.2) are recognized as highly refractory materials. Accordingly, shaped bodies of such materials, such as brick, are frequently employed in conjunction with high temperature processes. Zircon is more commonly the choice in view of its stability, ease of fabrication and lower cost.
One such high temperature process is production and/or depositing of metal oxides from vaporous reactants. Such oxides may be produced by flame hydrolysis or pyrolysis of a precursor in the form of a vapor, or carried by a vapor carrier. A widely used process is production of high quality fused silica by either pyrolysis or hydrolysis of silicon tetrachloride (SiCl.sub.4). Early patents disclosing such processes are U.S. Pat. Nos. 2,239,551 (Nordberg) and No. 2,272,342 (Hyde).
A commercial application of flame hydrolysis involves forming and depositing particles of fused silica to form large bodies (boules). Such boules may be used individually, or may be finished and integrated together into large optical bodies, such as telescope mirrors. In this procedure, SiCl.sub.4 is hydrolyzed, and the hydrolyzed vapor is passed into a flame to form molten particles of fused silica. The particles are continuously deposited on a bait or in a crucible, known as a cup, to form a boule.
The hydrolysis or pyrolysis of halides, in particular chlorides such as SiCl.sub.4, produces a very strong acid by-product, HCl. This by-product is highly detrimental to the processing equipment, including burners and deposition substrates. More important, however, it creates a very serious environmental problem, unless controlled by an emission abatement system. While such systems are available, they tend to be prohibitively expensive.
Accordingly, it has been proposed, in U.S. Pat. No. 5,043,002 (Dobbins et al.), to employ a halide-free, silicon-containing compound as a substitute for SiCl.sub.4. In particular, the patent proposes using a polymethylsiloxane, such as octamethylcyclotetrasiloxane, to provide the vaporous reactant for the hydrolysis or pyrolysis process.
Customer requirements make it imperative, in making such a process change, to maintain composition and/or physical properties of the resulting product essentially unchanged. Unfortunately, when the substitution proposed in the Dobbins et al. patent was introduced, changes did occur. In particular, a ten-fold increase in sodium ion contamination was encountered. Also, a substantial change occurred in the UV transmission of the fused silica produced.
Our present invention is predicated on providing remedies for the indicated problems. It is further predicated on providing improved equipment and process for producing fused silica by hydrolysis or pyrolysis of a vapor reactant. In particular, the invention is predicated on providing a method and means for producing a fused silica product that has a sodium ion contamination level below 100 parts per billion (ppb) while avoiding chloride emissions.